U.S. patent number 7,116,012 [Application Number 10/630,496] was granted by the patent office on 2006-10-03 for stable power conversion circuits.
This patent grant is currently assigned to General Motors Corporation. Invention is credited to Keming A. Chen, Lateef A. Kajouke, James F. Lazar.
United States Patent |
7,116,012 |
Kajouke , et al. |
October 3, 2006 |
Stable power conversion circuits
Abstract
A power conversion circuit includes a current section, a
transformer, and a voltage section. The current section includes
inductors configured to produce a boosted output voltage from a
voltage sourse when then current section is operating in a forward
direction. The transformer includes a primary winding electrically
coupled to the secondary winding and configured to drive a load.
The voltage section includes a pluralaty of balancing switchtes
configured to actively driven to provide a DC voltage to the load.
The balancing switches balance energy between the current section
and the voltage section when the power conversion circuit is the
load and operating in the forward direction.
Inventors: |
Kajouke; Lateef A. (San Pedro,
CA), Chen; Keming A. (Torrance, CA), Lazar; James F.
(Moorpark, CA) |
Assignee: |
General Motors Corporation
(Detroit, MI)
|
Family
ID: |
34103859 |
Appl.
No.: |
10/630,496 |
Filed: |
July 30, 2003 |
Prior Publication Data
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|
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Document
Identifier |
Publication Date |
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US 20050024904 A1 |
Feb 3, 2005 |
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Current U.S.
Class: |
307/64; 323/222;
363/15; 363/127; 363/60 |
Current CPC
Class: |
H02M
3/33569 (20130101); H02M 3/33592 (20130101); Y02B
70/10 (20130101) |
Current International
Class: |
H02J
7/00 (20060101) |
Field of
Search: |
;307/64 ;323/222
;363/15,60,127 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Sircus; Brian
Assistant Examiner: Cavallari; Daniel
Attorney, Agent or Firm: DeVries; Christopher
Claims
The invention claimed is:
1. A power conversion circuit comprising: a current section
comprising inductors and configured to produce a boosted output
voltage from a voltage source when the current section is operated
in a forward direction; a transformer having a primary winding
electrically coupled to said boosted output voltage and a secondary
winding; and a voltage section electrically coupled to the
secondary winding and configured to drive a load, said voltage
section comprising a plurality of balancing switches configured to
be actively driven to provide a DC voltage to the load and to
balance energy between said current section and said voltage
section when said power conversion circuit is not driving the load
and operated in the forward direction.
2. A power conversion circuit in accordance with claim 1 wherein
said current section comprises switched circuit branches, each said
branch having at least one said inductor and a branch switch, and
further comprising a control circuit configured to drive said
branch switches and said plurality of balancing switches.
3. A power conversion circuit in accordance with claim 2 wherein a
first subset of said plurality of balancing switches is driven in a
complementary fashion to a first subset of said branch
switches.
4. A power conversion circuit in accordance with claim 2 wherein
each of a plurality of subsets of said plurality of balancing
switches is driven in a complementary fashion to a corresponding
one of a plurality of subsets of said branch switches.
5. A power conversion circuit in accordance with claim 1 further
comprising a common ground, and wherein said current section is
electrically connected to said common ground and said voltage
section is electrically isolated from said common ground.
6. A power conversion circuit in accordance with claim 1 further
comprising the voltage source, and wherein said voltage source is a
rechargeable battery.
7. A power conversion circuit in accordance with claim 6 further
comprising the load, and wherein said load comprises a fuel cell
electrically coupled to the voltage section and configured to
receive an initial operating voltage therefrom.
8. A power conversion circuit in accordance with claim 7 wherein
said current section is configured to operate as a rectifier in a
reverse direction when said fuel cell is operating, and said
voltage section is configured to operate as a buck converter in a
reverse direction when said fuel cell is operating, to recharge
said rechargeable battery.
9. A power conversion circuit in accordance with claim 8 further
comprising a common ground, and wherein said current section is
electrically connected to said common ground and said voltage
section is electrically isolated from said common ground.
10. A power conversion circuit in accordance with claim 9 wherein
the common ground is a vehicle chassis.
11. A power conversion circuit in accordance with claim 8 wherein
said current section comprises switched circuit branches, each said
branch having at least one said inductor and a branch switch, and
further comprising a control circuit configured to drive said
branch switches and said plurality of balancing switches.
12. A power conversion circuit in accordance with claim 11 wherein
a first subset of said plurality of balancing switches is driven in
a complementary fashion to a first subset of said branch
switches.
13. A power conversion circuit in accordance with claim 11 wherein
each of a plurality of subsets of said plurality of balancing
switches is driven in a complementary fashion to a corresponding
one of a plurality of subsets of said branch switches.
14. A power conversion circuit comprising: a current section
configured to receive a input voltage from a voltage source at a
first node and common ground, said current section having a pair of
switched circuit branches each comprising an inductor and a branch
switch and configured to boost a voltage from the current source
when the current section is operated in a forward direction; a
transformer having a primary winding and a secondary winding, said
primary winding configured to receive said boosted voltage from
said current section when said current section is operated in the
forward direction, and wherein said secondary winding is
electrically isolated from said common ground; and a voltage
section coupled to said transformer, isolated from said common
ground, and configured to drive a load, said voltage section
including a plurality of actively-driven balancing switches
configured to balance energy between said current section and said
voltage section when said voltage section is operated in the
forward direction and said power conversion circuit is not driving
said load.
15. A power conversion circuit in accordance with claim 14 further
comprising a control circuit configured to switch said switched
circuit branches and to actively drive said plurality of balancing
switches.
16. A power conversion circuit in accordance with claim 15 wherein
a first subset of said plurality of balancing switches is driven in
a complementary fashion to a first subset of said branch
switches.
17. A power conversion circuit in accordance with claim 15 wherein
each of a plurality of subsets of said plurality of balancing
switches is driven in a complementary fashion to a corresponding
one of a plurality of subsets of said branch switches.
18. A power conversion circuit in accordance with claim 17 wherein
said current section is configured to operate as a rectifier when
operated in a reverse direction and said voltage section is
configured to operate in a buck mode when operated in the reverse
direction.
19. A power conversion circuit in accordance with claim 18 wherein
the voltage source is rechargeable and the power conversion circuit
includes said rechargeable voltage source.
20. A power conversion circuit in accordance with claim 19 wherein
the load comprises a fuel cell, and the power conversion circuit
includes said fuel cell.
21. A power conversion circuit in accordance with claim 20 wherein
said voltage section is configured to apply an initial voltage to
the fuel cell when operated in the forward direction, and to
recharge the voltage source from the fuel cell when operated in the
reverse direction.
22. A method for operating a power converter circuit having a
current section, a transformer, and a voltage section, the current
section having a plurality of switched inductors and branch
switches and the voltage section having a plurality of balancing
switches; said method comprising: utilizing the branch switches to
switch the switched inductors to generate a boosted voltage from a
voltage source; applying the boosted voltage to a primary winding
of the transformer to produce a transformed voltage; coupling a
transformed voltage from a secondary winding of the transformer to
a balancing section configured to provide power to a load; and
actively driving the balancing switches in the voltage section to
rectify the transformed voltage applied to the load, and to balance
energy between the current section and the voltage section when the
power converter circuit is not providing power to the load and is
operating in a forward direction.
23. A method in accordance with claim 22 further comprising
operating a control section to actively drive the balancing
switches and to switch said branch switches.
24. A method in accordance with claim 23 wherein said operating the
control section comprises driving a first subset of the balancing
switches in a complementary fashion to a first subset of the branch
switches.
25. A method in accordance with claim 23 wherein said operating the
control section comprises driving each of a plurality of subsets of
the balancing switches in a complementary fashion to a
corresponding one of a plurality of subsets of the branch
switches.
26. A method in accordance with claim 22 further comprising
operating the current section and the voltage section in a reverse
direction, and operating the voltage section in a buck mode and the
current section as a rectifier when operated in the reverse
direction.
27. A method in accordance with claim 26 further comprising
recharging the voltage source when operating the current section
and the voltage section in the reverse direction.
28. A method in accordance with claim 27 further comprising
utilizing the rectified, transformed voltage to apply a starting
voltage to a fuel cell.
29. A method in accordance with claim 28 performed in a
vehicle.
30. A method in accordance with claim 28 further comprising
operating a control section to actively drive the balancing
switches and to switch said branch switches.
31. A method in accordance with claim 30 wherein said operating the
control section comprises driving a first subset of the balancing
switches in a complementary fashion to a first subset of the branch
switches.
32. A method in accordance with claim 30 wherein said operating the
control section comprises driving each of a plurality of subsets of
the balancing switches in a complementary fashion to a
corresponding one of a plurality of subsets of the branch switches.
Description
FIELD OF THE INVENTION
The present invention relates to switching power conversion
circuits, including, but not limited to, bi-directional power
conversion circuits.
BACKGROUND OF THE INVENTION
Bi-directional, isolated DC-to-DC inverters are useful in a wide
range of applications. Examples of such applications include, but
are not limited to, uninterruptible power supplies, battery
charging systems, and auxiliary power supplies for hybrid
electrical vehicles.
It has been recognized that two-inductor boost converters suffer
from limited output voltage regulation range when operated at low
duty cycles. More particularly, when a load is below the minimum
input power of a converter circuit of this type, further decreases
in the load result in abnormal increases in output voltage due to
excess energy storage in the inductors. Yan et al. have proposed a
solution to avoid these increases in output voltage. ("Isolated
Two-Inductor Boost Converter with One Magnetic Core," Eighteenth
Annual Applied Power Electronics Conference and Exposition, Feb. 9
13, 2003, Miami Beach, Fla., pp. 879 885.) An auxiliary transformer
is utilized in series with two inductors to magnetically couple the
two input current paths, ensuring that the current in the two
inductors is the same. Thus, inductor current is eliminated when
the load draws no current. A magnetic component disclosed by Yan et
al. provides an isolated two-inductor boost converter with one
transformer. This component has two inductor windings intrinsically
coupled and is implemented with one gap in a three-leg magnetic
core. However, circuits utilizing an auxiliary transformer and
those utilizing the magnetic component of Yan et al. may require
that these windings or components be capable of carrying high
currents.
Another example of a DC-to-DC converter is disclosed by Li et al.
("A Natural ZVS High-power Bi-direction dc-to-dc Converter with
Minimum Number of Devices," presented at IEEE Industry Applications
Society Annual Meeting, Sep. 30 Oct. 4, 2001, Chicago, Ill., pp.
1874 1881.) This converter is operated with dual half-bridges
placed on each side of an isolation transformer. When power flows
from the low-voltage side to the high-voltage side, the circuit
operates in boost mode. Conversely, when power flows in the
opposite direction, the circuit works in buck mode to recharge a
battery used to provide power to the low-voltage section. To
support bidirectionality, a complex thirteen-step commutation
procedure is used that depends upon the magnitudes of various
currents at specified times.
SUMMARY OF THE INVENTION
Some configurations of the present invention therefore provide a
power conversion circuit having balancing switches that are
actively driven to balance energy in a no-load condition. For
example, in some configurations, a current section of the power
conversion circuit has switched inductors. The switched conductors
are configured to produce a boosted output voltage from a voltage
source when the current section is operated in a forward direction.
A transformer of the power conversion circuit has a primary winding
electrically coupled to the boosted output voltage and a secondary
winding. A voltage section of the power conversion circuit is
electrically coupled to the secondary winding and is configured to
electrically couple to a load. The voltage section includes a
plurality of balancing switches that are configured to be actively
driven. The balancing switches are configured so that, when they
are actively driven, a DC voltage can be provided to the load. In
addition, the driving of the balancing switches balances energy
between the current section and the voltage section when the power
conversion circuit is in a no-load condition and operated in the
forward direction.
In some configurations, the current section includes switched
circuit branches. Each branch has at least one switched inductor
and a branch switch. The power conversion circuit also includes a
control circuit that is configured to drive the branch switches and
the plurality of balancing switches. In various configurations, a
plurality of subsets of balancing switches are driven in a
complementary fashion to a corresponding subset of the branch
switches.
The current section, the voltage section, or both sections can be
(but need not be) isolated from a common ground. For example, in
some configurations, the current section is electrically connect to
a common ground, and the voltage section is electrically isolated
from the common ground.
Some configurations of the present invention employ a rechargeable
battery as a voltage source. (As used herein, a "battery" may
include one or more cells.) The load can be a fuel cell coupled to
the voltage section and configured to receive a initial operating
voltage from the voltage section. For example, a boosted voltage
from the battery can be used in some configurations to drive a
compressor that starts the operation of the fuel cell. Also, in
some configurations, when the fuel cell is operating, the current
section is configured to operate as a rectifier in a reverse
direction and the voltage section is configured to operate as a
buck converter. Such configurations can be used to utilize an
operating fuel cell to recharge the rechargeable battery, which can
be useful in various types of vehicles and other devices.
Various configurations of the present invention will be seen to
reduce the need for additional inductive components to stabilize
the power conversion circuit when operated in a no-load condition,
and provide substantial voltage boosts in an efficient manner.
Further areas of applicability of the present invention will become
apparent from the detailed description provided hereinafter. It
should be understood that the detailed description and specific
examples, while indicating the preferred embodiment of the
invention, are intended for purposes of illustration only and are
not intended to limit the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood from the
detailed description and the accompanying drawings, wherein:
FIG. 1 is a schematic diagram representative of various
configurations of no load controllers of the present invention.
FIG. 2 is a timing diagram of certain electrical signals in the
schematic diagram of FIG. 1.
FIG. 3 is a timing diagram of the electrical signals shown in FIG.
2 showing particular timing intervals useful in some
configurations.
FIG. 4 is a timing diagram of inductor current in a switching
branch of FIG. 1, corresponding to the electrical signal timing
intervals shown in FIG. 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following description of the preferred embodiment(s) is merely
exemplary in nature and is in no way intended to limit the
invention, its application, or uses.
In some configurations of the present invention and referring to
FIG. 1, a power conversion circuit 10 comprises a current section
22. Current section 22 is configured to receive an input voltage
from a voltage source E1 between a first node 12 and a node 14 that
can be a common ground. For example, voltage source E1 may be an
automobile battery supplying 9 to 15 volts DC. Common ground 14 may
be a vehicle chassis or a conductive portion thereof. The vehicle
chassis may be an automobile chassis, and voltage source E1 may be
a rechargeable storage battery. However, the present invention is
not limited to use in automobiles or other vehicles.
Current section 22 in some configurations comprises a pair of
switched circuit branches 16, 18 each of which comprises an
inductor L1, L2, respectively. (The proximity of inductors L1 and
L2 in FIG. 1 is not intended to indicate that inductors L1 and L2
have any appreciable mutual magnetic coupling.) Current section 22
is configured to boost a voltage from voltage source E1 when
operated current section 22 is operated in a forward direction. For
example, as is known in the art, circuit branch 16 can operate to
produce a voltage
##EQU00001## between node 20 and ground 14. In this expression,
V.sub.in is the voltage provided by voltage source E1. T is the
switching period (i.e., the time taken for one on/off cycle) of
branch 16 switch S1. t.sub.on is the "on" (i.e., closed) time of
branch 16 switch S1 during a cycle. To provide this boost, a
rectifier would be placed between inductor L1 and node 20. The
resulting DC voltage would be filtered and a load sufficient to
ensure stability would be placed across the filtered DC voltage.
Circuit branch 18, inductor L2, switch S2, and node 23 could
provide a boost in the same manner. However, in some configurations
of the present invention, an isolation transformer T1 and a set of
balancing switches are used in conjunction with circuit branches 16
and 18. For example, transformer T1 is coupled between nodes 20 and
23 on the one side and filtering capacitor CO and load RL on the
other.
Circuit branches 16 and 18 can be switched utilizing a control
circuit 24 configured to operate switches S1 and S2, respectively.
For example, control circuit 24 provides two control signals CS1
and CS2. Control signal CS1 operates to open and close branch 16
switch S1 and control signal CS2 operates to open and close branch
18 switch S2. In this manner, an alternating, boosted voltage is
applied to a primary winding 26 of an isolation transformer T1.
This boosted voltage is not isolated from common ground 14. More
particularly, whenever either or both of branch switches S1 and/or
S2 are closed, primary winding 26 of transformer T1 is electrically
coupled to ground 14.
In many configurations of the present invention, control circuit 24
includes logic (not shown) that provides a minimum duty cycle for
each switch S1 and S2 of 50 percent. Control circuit 24 also
prevents branch switches S1 and S2 from both being in the "open"
state at the same time. A timing diagram showing an example of the
timing of control signals CS1 and CS2 is shown in FIG. 2. FIG. 2 is
intended to be sufficiently illustrative to serve for explanatory
purposes, but should not be interpreted as showing timing range
limits or restrictions of any particular configuration. During
times when both switch S1 and S2 are closed, there is no voltage
across primary winding 26 of transformer T1.
Referring again to FIG. 1, isolation transformer T1 also has a
secondary winding 28 that is coupled to a voltage section 30 that
is electrically isolated from common ground 14. By "isolated from
common ground," it is meant that voltage section 30 is configured
to couple a voltage isolated from common ground to a load RL.
Control signals CS3 and CS4 may also be isolated from common ground
14, but need not be for voltage section 30 to be considered
"isolated from common ground." In various configurations, voltage
section 30 comprises a plurality of balancing switches S3, S4, S5,
and S6. Voltage section 30 rectifies a voltage from transformer T1,
produces a DC voltage therefrom, and is configured to electrically
couple this DC voltage to a load RL.
In various configurations of the present invention, a first group
of balancing switches are operated simultaneously with branch 18
switch S2. For example, balancing switches S3 and S6 are opened and
closed simultaneously with the closing and opening, respectively,
of branch 18 switch S2. Also, a second group of balancing switches
are operated simultaneously with branch 16 switch S1. For example,
balancing switches S4 and S5 are opened and closed simultaneously
with the closing and opening, respectively, of branch 16 switch S1.
Thus, there are a plurality of sets of switches for which a subset
of balancing switches on current section 22 is actively driven
(i.e., by a control current or voltage, such as CS3 or CS4, applied
to a control input of the switch, for example, a gate of an FET).
The subsets of balancing switches are driven in a complementary
fashion (oppositely and either simultaneously or nearly
simultaneously) to a subset of branch switches on voltage section
30. One such set comprises branch 16 switch S1 in current section
22 and switches S4 and S5 in voltage section 30. The other set
comprises branch 18 switch S2 in current section 22 and switches S3
and S6 in voltage section 30. The complementary operation of the
sets of switches results in the same polarity of voltage being
applied to load RL and smoothing capacitor CO, whether branch
switch S1 is closed and branch switch S2 is open or branch switch
S2 is closed and branch switch S1 is open. Thus, balancing switches
S3, S4, S5, and S6 operate as a switched full-wave bridge
rectifier, relative to the alternating voltage at the secondary of
T1, when voltage section 30 is operated in a forward direction. (As
used herein, power conversion circuit 10 is operated in a forward
direction when both current section 22 and voltage section 30 are
operated in their forward directions.)
In various configurations of the present invention, a control
circuit 24 is utilized to generate switching signals CS1 and CS2.
Also, inverters U1A and U1B are used to derive switching signals
CS4 and CS3 from CS1 and CS2, respectively, as shown in FIG. 1 and
FIG. 2.
Some configurations of the present invention are useful in
vehicular systems in which voltage source E1 is a rechargeable
battery and load RL is a compressor motor for a fuel cell 32.
Compressor RL requires an initial operating voltage to be supplied
by voltage section 30. In some configurations, compressor RL may
require an initial operating voltage of approximately 300 volts to
start, but the only power available at startup is power derived
from voltage source E1. Voltage source E1 itself may be a 9 to 15
volt rechargeable battery grounded to a common ground 14. Although
voltage source E1 may be grounded to a vehicle chassis 14 (or a
portion thereof), the relatively high voltage required to start the
compressor may be isolated from the chassis ground for safety
and/or other reasons. A high voltage may be required by compressor
RL in some configurations. Because the high voltage may be required
at high current, it can be difficult to efficiently provide the
needed voltage boost solely by utilizing a step-up transformer T1.
Therefore, in some configurations, efficiency is increased by
providing a portion of the needed voltage boost by transformer T1
and the remainder by operating current section 22 as a boost
converter. For example, E1 may provide a nominal 12 volts,
transformer T1 may provide a step-up factor of 6 in voltage, and
current section 22 may provide a boost of 4.2. The boost of current
section 22 may be realized utilizing duty cycles of about 76% for
branch switches S1 and S2 (i.e., switch S1 is closed 76% of the
time, and switch S2 is also closed 76% of the time.) The values
that apply to this example may be varied as needed to meet the
requirements of other configurations, taking into account circuit
and component tolerances.
A current IB is drawn from voltage section 30 in some
configurations of the present invention. Initially, IB=IL, where IL
is the current drawn by compressor RL (and any other loads, not
shown, coupled across the high-voltage output 34, 36 of voltage
section 30). When fuel cell 32 is operating, it supplies a current
IF. When current IF is sufficient to supply the entire current IL,
a "no-load" condition of power conversion circuit 10 occurs, i.e.,
IB=0 and the load draws no steady-state energy from voltage source
E1. At or near the no-load condition, the switching action
resulting from control signals CS1, CS2, CS3, and CS4 stabilizes
inductor current through L1 and L2 and thereby avoids instability
in current section 22. For example, in some configurations, the
switching waveforms shown in FIG. 3 result in a steady-state,
no-load inductor current (through L1, for example) as shown in FIG.
4. Moreover, the operation of balancing switches S3, S4, S5, and S6
in the timing relationship described herein with switches S1 and S2
serves to balance energy between current section 22 and voltage
section 30. Thus, energy is transferred back and forth in a
steady-state, stable fashion, i.e., voltages remain controlled and
stable in a manner similar to the case in which a nominal load RL
were drawing current. Without such balancing, the voltage would
tend to increase, typically resulting in protection circuitry (not
shown in the Figures) tripping and shutting down the circuit
10.
Current section 22 is bi-directional in some configurations. More
particularly, when fuel cell 32 is operating, a sufficient current
IF is available to allow a current to flow in the negative IB
direction, which is converted to an alternating current by voltage
section 30. In this configuration, voltage section 30 operates in
reverse as a buck converter. The alternating current is coupled
from secondary winding 28 of transformer T1 to primary winding 26.
(If T1 is configured as a step-up transformer in the forward
direction, it will operate as a step-down transformer in the
reverse direction.) Current section 22 then operates in reverse as
a rectifier to produce a DC voltage at a suitable level for
charging source E1. For example, the voltage in some configurations
is reduced to between 12 15 volts to charge a lead acid storage
battery E1. Isolation of the high voltage return is provided by T1.
Thus, in a configuration in which power conversion circuit 10 is
utilized in an automobile to start a fuel cell compressor RL, the
return voltage (300 volts) of fuel cell 32 is isolated from the
automobile chassis.
In some configurations, branch switches S1 and S2 can comprise
IXFN230N10 semiconductor switches available from IXYS
Semiconductors. Switches S3, S4, S5, and S6 can comprise
HGTH30N60B3D semiconductor switches available from Fairchild. Other
suitable switches may be substituted as a design choice. For
example, other semiconductor switches or other types of switches
may be used in some configurations as a design choice.
In some configurations of the present invention, one or more
voltage and/or current sensors (not shown) can be provided in
current section 22 and/or voltage section 30 to provide feedback to
controller 24. This feedback may be utilized to adjust the duty
cycles of control signals CS1 and CS2 as necessary to increase
stability and/or improve voltage regulation. For example, voltage
and/or current sensors may be placed at any combination of node 12,
branch 16, branch 18, node 34 and/or node 36.
Configurations of the present invention do not require that node 14
be electrically connected to a common ground, nor that voltage
section 30 be isolated from a common ground. Either or both of
current section 22 or voltage section 30 may be electrically
connected to a common ground, with those sides (if any) not
electrically connected to the common ground left "floating."
Stray coupling, parasitic effects, circuit imbalances, and/or
deviations of components from their ideal models may cause voltages
or currents described herein to deviate slightly from their ideal
values in many embodiments. Slight imperfections from nominal
values can be tolerated without loss of utility and without
fundamentally changing the operation of configurations of the
invention. This same spirit is to be applied in construing certain
terms used herein, such as "simultaneously" and "drawing no
current."
It will thus be appreciated that various configurations of the
present invention reduce the need for additional inductive
components to stabilize the power conversion circuit when operated
in a no-load condition. In addition, various configurations of the
present invention provide substantial voltage boosts in an
efficient manner.
The description of the invention is merely exemplary in nature and,
thus, variations that do not depart from the gist of the invention
are intended to be within the scope of the invention. Such
variations are not to be regarded as a departure from the spirit
and scope of the invention.
* * * * *